In various processes of culturing or fermenting microorganisms, it is sometimes necessary during or at the conclusion of the fermentation process to kill active cells in the mixture. This is particularly true when microorganisms containing recombinant DNA are grown as production hosts and it is desirable to prevent any viable recombinant organisms from being released into the environment. Even if the microorganisms do not contain recombinant DNA, it is often desirable to kill the cells prior to processing in order to ensure that viable cells are not released to the environment either in the product or in the waste products of the process.
Many conventional methods required to kill microorganisms, such as heat, are too severe and may destroy or alter the desired secreted product before the cells are killed. In this case, the product must be recovered without killing the cells, which requires the use of tedious and costly containment procedures and equipment. U.S. Pat. Nos. 5,801,034 and 5,378,621 describe a method for killing microbial cells with a single organic acid having 1 to 5 carbon atoms. However, according to the examples of these patents, a high level of organic acid and a low pH is optimal. The low pH condition is often detrimental to the stability of many enzyme products of interest in the fermentation medium and the high level of organic acid is often inhibitory to downstream applications in which it is desirable to use the enzyme products, such as fermentation of a microorganism that produces an organic substance on a substrate that has been produced by enzymatic catalysis with the enzyme product. Further, use of a high concentration of chemical agent for killing microbial cells may add significantly to the cost of the product recovered from the fermentation medium. Accordingly, there remains a need to develop methods of killing cells under less harsh conditions and with a lower concentration of chemical agents would be desirable.